1. Field of the Invention
This invention relates to an implant including an electric power consuming device connected to an electric power source with an anode and a cathode. This invention also relates to such an implant which consumes electric power itself and/or which is coupled to a separate, implanted device which consumes electric power.
2. Description of Related Art
In currently available implants which require an electric power source for operation, for example, cardiac pacemakers, hearing aids, stimulation devices and the like, either primary cells or secondary cells are used as the electric power source. A drop in the efficiency of the electric power source which endangers implant operation can be prevented by replacing or recharging the cell before the expected service life of the electric power source expires. However, because any replacement of the electric power source requires surgery on the implant wearer, achieving a long service life of the electric power source is very important and is of the highest priority in the field of implant technology.
In order to be able to predict the efficiency of the electric power source provided in the implant, whether it be a primary or secondary source, and to also prevent processes that damage the individual electrodes which can occur especially in charging processes of an electric power source made as a secondary cell, the electrodes should be monitored with respect to certain characteristics such as current and voltage.
In an implant equipped with a conventional electric power source, the electrodes of the electric power source cannot be observed and monitored independently from the other electrode. Rather, the characteristics of current and voltage which can be measured outside the power source, are always referenced to the entire combination of the electrodes provided in the electric power source. When these characteristics are measured, they are generally dependent on the fact that these electrodes have predictable properties during discharge, at rest and optionally, during charging. However, this measurement can be adulterated by simultaneous processes which polarize the electrodes differently. Thus, this measurement allows conclusions regarding the instantaneous state of the electric power source only under current conditions and only with accurate knowledge of the simultaneous processes under the boundary conditions prevailing at the time.
For example, when charging a secondary electrochemical cell, the equilibrium potentials of the two active electrodes are shifted to more negative (negative electrode) and more positive (positive electrode) potentials due to the existing internal resistances. The internal resistances are thus composed of ohmic and non-ohmic portions. The ohmic portions generally relate to contact and electrolytic resistors. The non-ohmic portions are dictated by the electrode composition and geometry and the electrochemical processes which take place on the electrodes.
Overall, there is a very complex network of resistive, capacitive and inductive components which can no longer be broken down especially when there is loading, i.e. when the electric power source supplies the implant with electrical energy. Therefore, a simple current/voltage measurement cannot provide the basis for concluding which of the electrodes involved behaves as desired and which does not.
Only by extensive experience with a given system under clearly defined boundary conditions (for example, "discharging at C/2 rate to an end discharge voltage of 1.5 V"; "charging at C/10 rate for 14 h") can one skilled in the art assess whether the electric power source being tested is "good" or "bad" from simply measuring current and voltage values. In addition, even if the discharging behavior is known for a certain current load with a certain cut off criterion for a given electric power source, one skilled in the art still cannot exactly predict the behavior of the electric power source under different conditions, for example, at 1/10 or 1/100 of the current load at the known boundary conditions. At best, one skilled in the art can only give an estimate.